In Patch 2.4.0 the restrictions on warlock summoning were lifted, and thus those previous players that were unable to enter (unable to fly), may now enter depending on the minimum level for these instances.

In Patch 3.2.0 another barrier to entry was lowered when flying mounts (or Flight Form) became available at level 60.

Since Patch 3.3.0 it is no longer necessary to fly to the Tempest Keep, if the player uses the cross-realm Dungeon Finder tool (the player gets summoned to the instance automatically).